1. Field of the Disclosure
The present disclosure relates to a light multiplexer provided with a diffraction grating having a plurality of elongated grooves for reflecting or transmitting a light beam, formed on quartz or the like in parallel to each other and, more particularly, to a light multiplexer capable of preventing a grating pitch in a diffraction grating from being changed due to a change in temperature.
2. Discussion of the Background Art
A light multiplexer has a diffraction grating having numerous vertical grooves regularly arranged on a glass substrate in parallel to each other. When light beams are incident into the diffraction grating, the transmitted or reflected light beams are diffracted and interfere with each other, so that only the light beam having a specified wavelength can be taken out. Utilizing this fact, the light multiplexer multiplex or demultiplex the light beams. Here, a diffraction grating for reflecting a light beam is referred to as a reflection type whereas a diffraction grating for transmitting a light beam is referred to as a transmission type.
A light multiplexer in the related art is disclosed in, for example, Japanese Patent Application Laid-Open No. 9-73020. This light multiplexer is of a reflection type. As shown in FIG. 1(a), an optical waveguide array device 3 having an input/output array port 2 disposed at one end thereof, a lens 4, and a diffraction grating 5 are mounted on a substrate 1. The optical axes of the members are adjusted in predetermined states at the time of the mounting. Moreover, as shown in FIG. 1(b), the diffraction grating 5 is formed into a plate, and is securely supported on both sides thereof opposite to each other by rod-like supporters 6 and 7.
In the light multiplexer having the above-described configuration, a light beam emitted from the other side of the optical waveguide array device 3 is guided to the diffraction grating 5 through the lens 4, to be diffracted thereat. The diffracted light beam is incident into the other side of the optical waveguide array device 3 through the lens 4, to be thus multiplexed or demultiplexed.
The diffraction grating 5 used in the light multiplexer in the related art disclosed in Patent Document 1 is mostly made of a glass material such as quartz. In addition, each of the rod-like supporters 6 and 7 for securing the diffraction grating 5 onto the substrate 1 also is made of a glass material having a thermal expansion coefficient approximate to that of the diffraction grating 5. However, the substrate 1 is generally made of metal such as Kovar, and therefore, it has a different thermal expansion coefficient from those of the supporters 6 and 7 mounted directly on the substrate 1 and the diffraction grating 5.
As a consequence, when the temperature of the substrate 1 is increased, a portion of each of the supporters 6 and 7 in contact with or near the upper surface of the substrate 1 is more largely displaced than portions remote therefrom, as indicated by arrows Y1 and Y2 in FIG. 1(c). When each of the supporters 6 and 7 is displaced in such a manner, the diffraction grating 5 secured on both sides thereof to the supporters 6 and 7 also is displaced accordingly. In other words, since a grating pitch also is displaced in an X direction according to the displacement, the diffraction angle of the light beam is changed, thereby raising a problem that the light beam cannot be properly multiplexed or demultiplexed. The diffraction grating 5 is secured on both sides thereof to the supporters 6 and 7, and therefore, it is displaced in association with the displacement of the supporters 6 and 7 on both sides.
The change in diffraction angle caused by the temperature will be further explained with reference to a light multiplexer in the case where the reflection type diffraction grating 5 shown in FIGS. 1(a) to 1(c) is replaced by a transmission type diffraction grating 5-1 shown in FIG. 2(a). Here, only a center axis of an optical flux is indicated by a straight line between an optical waveguide input port 8 and the diffraction grating 5-1.
A light beam emitted from an optical waveguide input port 8 is transmitted to the diffraction grating 5-1 through a lens 4a, to be demultiplexed in an X direction at a predetermined wavelength interval by diffraction at the time of the transmission. The demultiplexed light beams are incident into another lens 4b at predetermined diffraction angles. Optical axes are refracted at an angle perpendicular to a waveguide forming end face of an optical waveguide output port 9 by the lens 4b, and then, the light beams are focused on the optical waveguide output port 9.
Here, when α represents an incident angle into the diffraction grating 5-1 whereas β represents a diffraction angle, the diffraction angle β is expressed by the following equation (1) according to a diffraction grating equation.d(sin α+sin β)=mλtherefore, β=arcsin(m·λ/d−sin α)  (1)where d represents a grating pitch; m, the order of diffraction; and λ, an optical wavelength.
According to the equation (1), it is found that the change in grating pitch in the diffraction grating 5-1 leads to the change in diffraction angle β.
In addition, when λ0 represents a center wavelength whereas dλ represents a wavelength interval, adjacent wavelengths may be designated by λ0+dλ and λ0−dλ, as illustrated in FIG. 2(b). In contrast, a light incident interval P into the optical waveguide output port 9 is obtained by the following equation (2) when f represents a focal distance of the lens 4b and dβ represents a difference in diffraction angle between the adjacent wavelengths:P=tan(dβ)·f  (2)
According to the equation (2), it is found that the change in difference dβ in diffraction angle changes the light incident interval P. Intrinsically, the light incident interval P is fixed, and therefore, when the difference dβ in diffraction angle is changed, a focused position OP1 of the light beam emitted from the lens 4b is shifted in the X direction with respect to a waveguide incident portion 9a at the end face of the optical waveguide output port 9, as illustrated in FIG. 2(c), thereby causing a coupling loss. In other words, the change in grating pitch in the diffraction grating 5-1 causes the coupling loss. Consequently, there has arisen a problem that the change in grating pitch in the diffraction grating 5-1 according to the change in temperature increases the coupling loss.
In order to solve the above-described problem, an object of the present disclosure is to reduce the displacement in grating pitch in the diffraction grating according to the change in temperature so as to reduce the change in diffraction angle of the light beam, thus properly multiplexing or demultiplexing the light beam.